Distributed energy resources (DERs) play an ever-increasing role in electrical power generation. Generally, a DER is a relatively low-power (relative to utility-scale fossil fuel, hydroelectric and nuclear power plants) electricity-generating or storage device that is connected to an electrical power system (EPS), for example, a utility power grid, to form distributed energy system.
Examples of DERs include diesel engine-generators, wind turbines, solar cells, fuel cells, backup batteries, and any combination of these. DERs are used for a number of reasons. For example, in some applications DERs are used to provide backup power when primary power from a utility grid is interrupted. In other applications DERs are used to reduce reliance on electrical power from a utility power grid. In still further applications, DERs are used to supplement power provided by conventional electrical power generators, such as fossil-fuel-fired and nuclear-fission driven power plants. DERs are becoming increasingly popular as more manufacturers are building DERs that generate electricity from renewable resources, such as wind and solar energy.
The DERs can be connected to a single or three phase network, according to their power ratings. In an alternating current (AC) distribution grid, DER can generate one or two components of total power, i.e., an active power (symbolized as P) and a reactive power (symbolized as Q). Reactive power is the component of electric power that does not produce any work, and signifies the phase angle at which DER's supply their current to the grid.
The reactive power generated or absorbed by the DER influences the state of the grid. Specifically, the amount of reactive power is proportional to the voltages in the grid, i.e., when more reactive power is generated, the voltages in the grid increase and when less reactive power is generated, the voltages decrease.
The capability of DER to provide active or reactive power to the grid is limited and may vary over time. There are potentially thousands of DER devices connected to the grid, and every DER has to cooperate to provide a total power (sum of active or reactive powers) required by the grid at a specific time. Accordingly, there is a need for a method by which each DER determines the amount of power to supply to the grid, such that a total number of DERs generates the target amount of power, subject to restriction that each DER remains within its capability bounds for the generation of the power.
Some conventional methods for this power sharing problem in smart distribution grids include methods using local measurements at each DER to assist in calculating its power output. However, those methods do not consider the capacities of other DERs and, are inaccurate, see, e.g., U.S. Patent Publications 2010/0117606, and 2012/0105023.
Some other methods assume full communications, where the DERs communicate with each other or with a centralized entity that specifies the power of each DER to meet the target is met, and then dispatches that command to each DER. This method suffers from communication overhead and is too slow for some situations. See, e.g., U.S. patent and patent Publications U.S. Pat. No. 7,508,173, U.S. 2010/0067271, U.S. 2012/0205981, U.S. 2012/0235498, U.S. 2012/0310434, U.S. 2013/0018516.
Another approach is based on partial communication between each DER and its neighbors. However, the current methods are asymptotical, i.e., each DER converges to a solution asymptotically without ever achieving the exact value of the power to be generated.
Accordingly, there is a need to provide a method for determining, within a finite number of communication steps, the power to be generated by each distributed energy resource (DER) into a power grid, such that the power generated by all DERs connected to the power grid meets the total power requirement of the power grid without violating the constraints of each DER.